Monday, February 15, 2021

Atmospheric Losses in satellite communications

 Atmospheric Losses
Losses occur in the earth’s atmosphere as a result of energy absorption by the atmospheric gases. These losses are treated quite separately from those which result from adverse weather conditions, which of course are also atmospheric losses. To distinguish between these, the weather-related losses are referred to as atmospheric attenuation and the absorption losses simply as atmospheric absorption.
The atmospheric absorption loss varies with frequency.  
For an elevation angle of 90° at the earth-station antenna, denoting this value of absorption loss as [AA]90 decibels, then for elevation angles down to10°, an approximate formula for the absorption loss in decibels is
 



Where El is the angle of elevation. An effect known as atmospheric scintillation can also occur. This is a fading phenomenon, the fading period being several tens of seconds. It is caused by differences in the atmospheric refractive index, which in turn results in focusing and defocusing of the radio waves, which follow different ray paths through the atmosphere. It may be necessary to make an allowance for atmospheric scintillation, through the introduction of a fade margin in the link power-budget calculations.


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